Device processing involving an optical interferometric thermometry using the change in refractive index to measure semiconductor wafer temperature

ABSTRACT

A method for fabricating a semiconductor device, which involves a technique for monitoring the temperature of the semiconductor substrate in which the device is formed, is disclosed. In accordance with the inventive technique, light, to which the substrate is substantially transparent, is impinged upon the substrate, and the intensity of either the reflected or transmitted light is monitored. If, for example, the intensity of the reflected light is monitored, then this intensity will be due to an interference between the light reflected from the upper surface of the semiconductor substrate and the light transmitted through the substrate and reflected upwardly from the lower surface of the substrate. If the temperature of the substrate varies, then the optical path length of the light within the substrate will vary, resulting in a change in the detected intensity. By comparing the detected intensity with intensities corresponding to known temperature variations, the temperature of the substrate is readily determined.

This application is a continuation of application Ser. No. 07/400,215,filed on Aug. 29, 1989, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the fabrication of devices and, in particular,to the fabrication of devices by thermal processes.

2. Art Background

Various methods have been devised for measuring the temperature of anarticle. Approaches generally rely on either optical or electricalmeasurements with or without a thermal probe. Generally electricalmeasurement techniques use a probe such as a thermocouple where theelectrical characteristics of a bimetal contact in proximity to thearticle is indicative of article temperature.

It is possible to perform optical measurements with a probe or directlyon the article without a probe. Exemplary of optical probes is a silicaoptical fiber including a terminal region of silicon. (See U.S. Pat. No.4,437,761 dated Mar. 20, 1984). The refractive index of the siliconregion varies strongly with temperature and thus light traversing theoptical fiber and incident on the silicon region is reflected both atthe interface of the silicon region with the silica fiber and at theinterface of the silicon region with the ambient. The resultinginterference pattern between the two portions of reflected light, due tothe strong temperature dependence of the refractive index in silicon,and to a significantly lesser extent to thermal expansion, allows ameasure of temperature. However, it must be assumed that the electricalor optical probe and the article are at the same temperature. Even whenthe probe contacts the article to be measured, such assumptions areoften at best approximate.

As discussed, direct measurement techniques not requiring the assumptionof temperature equivalence between probe and article are also available.Exemplary of these techniques is a process described by D. Hacman inOptik, 28, 115 (1968). In this technique, the temperature of a quartzsubstrate is monitored by directing visible light onto the surface ofthe substrate. Likewise, R. A. Bond, et al. in Journal of Vacuum Scienceand Technology, 18, (2), 335 (1981) have used this technique to measurethe temperature of a quartz substrate in a plasma reactor. As in thepreviously described optical fiber technique, interference occurs due toreflection at both the incident surface of the glass and at the remotesubstrate surface. Since the coefficient of linear expansion of thesubstrate is temperature dependent, a monitoring of the interferencepattern gives a measure of the change in substrate thickness and thusthe associated temperature change. Similarly, pyrometric techniques arealso available which do not depend upon an assumed equivalence betweenarticle and probe temperatures. In these measurements, black bodyradiation characteristic of temperature is emitted by the article and isdetected.

The quality of devices such as optical devices, electronic devices andoptoelectronic devices depends, to a large extent, on the control ofprocesses employed in their fabrication. A significant processingcondition in most such procedures is the temperature. For example, indeposition techniques where a heated substrate is subjected to gasesthat undergo thermally induced chemical reactions to produce depositionon the substrate, the substrate temperature significantly affects thecomposition of the resulting deposit. Exemplary of such depositiontechniques are molecular beam epitaxy (MBE), chemical vapor deposition(CVD) and metal organic chemical vapor deposition (MOCVD). (Adescription of these processes can be found in Chang and Ploog,Molecular Beam Epitaxy and Heterostructures, Martinus NijhoffPublishers, Dordrecht, 1985, Journal of Crystal Growth, 55 (1981) andChemical Vapor Deposition for Microelectronics by A. Sherman, Noyes DataCorporation, Parkridge, N.J., 1987. In general, these techniques alldepend on the interaction of gas phase entities with a heated substrateto produce deposition.) Similarly, etching processes such as plasmaetching and reactive ion etching (RIE) also depend on substratetemperature. For example, if the temperature across a wafer variessignificantly, the etch rate across the substrate also differs. Clearly,a spatial variation in etch rate across a substrate will produceundesirable nonuniformities during fabrication.

Presently, for processes such as MBE, MOCVD, and CVD the more accuratethe substrate temperature measurement the better the control of theprocess. Techniques such as plasma etching and reactive ion etching(RIE) can be advantageously controlled presently without temperaturemonitoring. However, as device structures become smaller, the effects oftemperature are expected to produce unacceptable nonuniformities even inthese etching techniques. Therefore, precise temperature monitoring isquite significant.

As a result, techniques relying on the weak assumption of temperatureequilibration between the substrate and temperature probe are notdesirable. Techniques such as optical pyrometry also involve significantinaccuracies. Optical pyrometry depends on the measure of absoluteintensity of light emitted by a substrate. This absolute intensity isstrongly affected by properties such as transmittance of optical windowsin the fabrication chamber and emissivity of the substrate itself. Sinceit is generally expected that these parameters will significantly changeduring processing, i.e. unavoidable contamination will be expected tochange window transmission and substrate surface change will be expectedto effect substrate emissivity, the measure of absolute intensity isinaccurate at best.

Techniques involving the monitoring of changes in the linear coefficientof expansion with temperature, although initially suggested formonitoring of device processing, have not been pursued. This lack ofactivity has possibly occurred due to inaccuracies inherent in therelatively small change of expansion coefficient with temperature.Irrespective of the reasons, a satisfactory technique for temperaturemonitoring associated with device processing is not presently available.

SUMMARY OF THE INVENTION

The invention involves a technique for directly measuring thetemperature variation of a semiconductor substrate, such as a siliconwafer, during one or more temperature-sensitive processing steps leadingto the fabrication of a semiconductor device. Significantly, theinventive technique avoids the need to achieve good thermal contact tothe substrate, as is the case when using thermocouples, and also avoidsthe need for frequent calibrations, as is the case with conventionalpyrometry.

In accordance with the inventive technique, the upper and lower surfacesof the substrate are made reflective, and light, to which the substrateis substantially transparent, is impinged on one of the surfaces, e.g.,the upper surface. Then, the intensity of either reflected ortransmitted light is monitored as a function of time, and thus oftemperature, passing at some point through a known temperature. Formonitoring reflected light a portion of the incident light is reflectedfrom the upper surface, and another portion is transmitted, i.e.,refracted, through the semiconductor substrate. Upon impinging on thelower surface, part of the refracted portion is reflected upwardlythrough the semiconductor substrate, exiting the substrate to interferewith the light which was reflected from the upper surface. Formonitoring transmitted light, at least a portion of the incident lightis transmitted through the thickness of the semiconductor substrate. Atthe lower surface, some of this light is transmitted and some isreflected to the upper surface, where it is again reflected downwardlyto interfere with the transmitted light. In either case, as thetemperature of the substrate varies during processing, the optical pathlength of the light within the substrate will vary, resulting indiffering degrees of constructive and destructive interferences. As aconsequence, the intensity of the detected light will vary as a functionof temperature, and thus of time. By comparing this intensity variation,called an interferogram, with a calibrated interferogram, thetemperature of the substrate is readily determined. The devicefabrication process is controlled based on the determined temperature.

It should be noted that in the case of semiconductors, variations intemperature have a much larger effect on the thermal refractive indexcoefficients that on the corresponding thermal expansion coefficients.As a consequence, it is the variations in the refractive index (withtemperature) which largely determine the variations in optical pathlength of the light within the semiconductor substrate.

DETAILED DESCRIPTION

As discussed, the invention involves a process for fabricating asemiconductor device including at least one processing step whichdepends on temperature and where this temperature is monitored tocontrol the process by measuring the optical path length of thesemiconductor material.

The procedure is generally applicable to fabrication processes involvingsemiconductor materials, i.e., materials having a bandgap less than 1.9electron volts such as Si, Ge, InP, GaP, CdTe, InSb and GaAs. Generallysuch materials have a strong temperature dependency, i.e. greater than50% of the change in optical path length with temperature being due tochange in refractive index. Exemplary processing steps include etchingand various types of deposition procedures such as MBE, MOCVD, and CVD.

The temperature measuring procedure involves illumination of thesemiconductor material from a light source capable of creatinginterference (e.g., laser light) and measurement of the reflected ortransmitted optical intensity. The light being used is generally in thewavelength range 600 to 10,000 nm and should be chosen so that no morethan 90% of the incident light is absorbed in the substrate. Thesubstrate being processed should have essentially parallel surfaces(i.e. should, over any region of 10 μm dimension in the monitored area,have thickness variation of less than the wavelength of light being usedfor monitoring) and the opposing surface should be sufficientlyreflective (typically at least 3% reflective) to produce a signal ofsufficient intensity to be detected. Generally the power of the incidentlight should be greater than 10⁻⁶ W to produce a detectable signal.Additionally, power densities greater than those producing a 2° C.temperature rise should be avoided. This power density is, for example,approximately 10⁵ W/cm² in a 0.1 mm diameter spot at a wavelength where90% of the light is absorbed in a single pass through the substrate.Typically, for surfaces of at least 3% reflectivity observed intensitiesof about 10⁻⁶ W in transmission and of about 10⁻⁸ W in reflection areobtained. The intensity of the reflected or transmitted light beam ismonitored with, for example, photodiodes, photomultipliers, orcharge-coupled devices. The intensity variation data is compared with:either 1) that expected from the temperature change in optical pathlength expected theoretically as determined in accordance withcalculations for the intensity of reflected or transmitted light asdescribed by M. Born and E. Wolf in Principle of Optics (Pergamon Press,New York, 1980) combined with calculations for refractive indexdependency on temperature as described by F. Stern in Phys. Rev. A, 133,1653 (1964) and thermal expansion dependency on temperature as describedby Y. S. Touloukian, et al, Thermophysical Properties of Matter, 13,"Thermal Expansion", Plenum Press, New York, 1977; or 2) that obtainedfrom calibration measurement. The calibration measurements are typicallymade by mounting a small substrate and a thermocouple in an isothermalenclosure and slowly heating the enclosure while simultaneouslymonitoring transmitted light and thermocouple temperature, therebyacquiring a calibrated interferogram.

In a specific embodiment of the invention, a semiconductor wafer(substrate) being processed, such as a 0.5 mm thick silicon wafer, has apair of polished essentially parallel surfaces. (Typical wafers have ataper angle generally between 0.01° and 0.1° over a 1 mm diameter areato be probed by the incident light.) The light source, e.g. laser, suchas a 1.5 micron indium gallium arsenide phosphide laser or a helium-neonlaser emitting a wavelength of about 1.15 microns or 1.52 microns, isselected such that its output radiation has a wavelength for which theabove mentioned surface reflectivity is attained.

During device fabrication, a semiconductor substrate is subjected to anumber of processing steps (as previously discussed), such as materialformation, doping with impurities and etching. The invention involves aprocedure where, at least one of these processing steps is dependent onthe temperature of the wafer. This temperature dependent step istypically performed in a closed container, at least a portion of whosesurface has a transmissivity of at least 3% to the monitoring radiation.While the wafer is being processed in accordance with the aforementionedtemperature dependent step, radiation from the laser is directed throughthe transparent portion of the container onto the wafer and theradiation reflected by or transmitted through the wafer is observedthrough the transparent portion of the surface of the container--as inan interferometric measurement configuration.

In accordance with known principles of optics, an interference intensityis observed. (If the substrate region being monitored has a tapertypically greater than 10⁻³ degrees per cm of beam diameter then aseries of spatial fringes relative to the difference in optical pathlength will be observed.) As the temperature varies the intensityvaries. If a spatial fringe pattern is present across the transmitted orreflected beam, this pattern of fringes traverses the beam profile in adirection normal to the direction of propagation of the reflected light.For the case of a substrate tapered such that the thinner end is towardthe right, observation of the direction of motion of the fringes (ifpresent)--from left to right versus right to left--is used to determinewhether the temperature of the substrate is increasing versusdecreasing. To determine the absolute temperature change, in thesituation with no fringes the calibration data is employed together withdetermination of temperature change reversal through observation ofanomalously low intensity maximum or anomalously high intensity minimumas temperature varies.

In an exemplary calibration procedure, the test wafer is placed in aboron nitride cavity which is subjected to various temperature changes(measured by a thermocouple or platinum resistance thermometer) andirradiated with laser radiation. The laser radiation (typicallyimpacting the wafer at an angle to the wafer of greater than 10° withrespect to the wafer surface) after reflection by or transmissionthrough the wafer is observed through a hole in the cavity. In this way,a calibration standard for the temperature of the wafer being processedis established. Advantageously, the temperature range over which thetest wafer is subjected, includes the same temperature range as thatwhich the wafer to be processed is subjected.

We claim:
 1. Method of manufacturing a semiconductor device, the methodcomprisinga) providing a semiconductor body having a first and a secondmajor surface, b) carrying out one or more processing steps on the body,at least one of the steps depending on the temperature of the body;characterized in that the method further comprises c) exposing the bodyto electromagnetic radiation and detecting radiation reflected from, ortransmitted through, the body wherein said radiation undergoesinterference; d) monitoring the intensity variation with time through anextremum of the detected radiation as the temperature of the bodychanges during one of said processing steps to determine a valuerepresentative of said temperature based at least in part on the changeof refractive index of said body with said temperature, the processingstep to be carried out at a nominal processing temperature T_(p) ; ande) controlling at least one of said processing steps in accordance withthe result of step d).
 2. Method of claim 1, wherein the methodcomprises comparing the intensity of the detected radiation with a knowncalibration target intensity associated with the temperature T_(p). 3.Method of claim 1, wherein both major surfaces of the body arerelatively smooth such that each surface specularly reflects at least 3%of said radiation that is incident thereon.
 4. Method of claim 1,wherein the semiconductor body is a semiconductor wafer, and wherein thesemiconductor comprises a material selected from the group consisting ofSi, Ge, GaAs, InP, GaP, CdTe and InSb.
 5. Method of claim 1, whereinsaid radiation is in the wavelength range 600 to 10,000 nm, and whereinthe radiation is laser radiation.
 6. Method of claim 1, wherein theintensity of the detected radiation is a function of the optical pathlength of the detected radiation in the semiconductor body, the opticalpath length being a function of the temperature of the body, withgreater than 50% of the change of optical path length with temperaturebeing due to a change of the refractive index of the body withtemperature.
 7. Method of claim 1, wherein a portion of the first majorsurface is exposed to the radiation, the portion being substantially butnot exactly parallel to the second major surface, such that a pluralityof spatial interference fringes are formed, a change in the temperatureof the body resulting in a movement of the fringes; wherein the methodcomprises sensing the direction of movement of the fringes, whereby thesense of the temperature change can be determined.
 8. Method of claim 1,wherein the semiconductor body comprises a relatively thick firstsemiconductor substrate and at least one relatively thin secondsemiconductor layer, the second layer differing in composition and/ordoping level from the first layer, step b) including changing thethickness of the second layer, wherein a property of said layer dependson the temperature of the body.
 9. Method of claim 1, wherein saidradiation is laser radiation.